What are the intentions of the different forms of Armature/Bone Parent/Child relationships?
Parent Child Relationship and Keep Transform




There are different ways to create a Parent/Child relationship in Blender or parent.  
Keep Transform stealthily does the appropriate math with matrices to preserve child placement.
Some ways have [immediate] effect in placement, others (Keep Transform) [delayed].







In the two object parent and armature parent menus above we see (Keep Transform) as a valuable option.
The [Keep Transform] genre intentionally preserve child placement at the time of parent assignment, then you move the Parent for different results.  
You should choose the one the best suits your context.  If the [immediate] effect is not what you want today, then the suggestion is to view it as a feature that you may use in the future for a positive Blender User experience.
Both parent menus above have many different options so you may expect this as a feature.  All of those options may be useful to Blender Users in some particular cases.
Via Control-P use Keep Transform for the case in question.  Manually filling in Parent does not necessarily [Keep Transform].  If the parent has any (non default) Scale, Rotations, or Translation the new Child will be transformed accordingly.  Expect this designed behavior.  It is a feature not a defect.
This was intended as a short explanation.  See more here at BSE or the online Blender Manual